Cement kiln dust (CKD) is generated by cement kilns worldwide. For example, CKD is generated in cement kilns during the production of cement clinker. Generally, CKD is a particulate mixture, including, amongst its constituents, partially calcined and unreacted raw feed, clinker dust and ash, enriched with alkali sulfates, halides, trace metals and other volatiles.
CKD varies significantly dependent upon the specific plant process and raw materials. For reference purposes a CKD composition as reported by the Bureau of Mines* includes:
Constituent% by WeightCaCO355.5%SiO213.6%CaO 8.1%K2SO4 5.9%CaSO4 5.2%Al2O3 4.5%Fe2O3 2.1%KCl 1.4%MgO 1.3%Na2SO4 1.3%KF 0.4%Others 0.7%* Typical Composition of Cement Kiln Dust (Haynes and Kramer, 1982)
CKD may be captured in a particulate collection system such as a baghouse or electrostatic precipitator. The captured CKD may then be utilized in a number of manners. For example, the captured CKD may be placed back into the kiln process as a raw material; placed into the cement milling process as a process addition; sold as a cementitious material; and/or sent to a landfill.
The properties and composition of CKD can be significantly affected by the design or operation of, or the materials used in, a cement kiln, with the result that constituent chemical and physical characteristics of CKD must be evaluated on an individual plant basis. In general, the alkaline nature of the CKD makes it a good neutralizing agent when adsorbing metals.
The CKD which is returned to the kiln on a routine basis undergoes a clinkering and/or calcination process, which causes compounds and trace elements to be volatilized. These materials are released back into the kiln gas stream. Over time, this often creates a cycle within the kiln gas stream, building up concentrations of the elements and compounds in the CKD which do not readily combine into clinker. As a result of the cement manufacturing process, materials such as mercury and other heavy metals may continue building in concentration. A portion of these elements may be removed from the gas stream through the scrubbing effect of the raw mill, but only when it is in suitable operation. For example, pulverized limestone may absorb at least a portion of volatilized or other trace materials directly from the exhaust gas stream which is used for drying in a raw mill. However, when the raw mill is shut down, or when cement kiln or raw mill operations are otherwise unable to absorb sufficient amounts of elements of the exhaust gas stream, greater quantities or concentrations of these elements may be released to the atmosphere through the cement kiln exhaust stack or absorbed by the CKD. Therefore, it is often desirable to treat the CKD before it is recycled back into the system.